Triangle Tube Challenger 105 DHW problem

Does anyone have an idea why the DHW does not seem to sense a call for heat on the TT Challenger? I don't see that there are many installed as the Prestige is a more popular model so I have not seen much posted about problem solving for this unit.
I have been using just the hot water loop on this combi for a few weeks without issues. My system was designed with a hot water return loop that was not initially set up. So I installed a pump that joins the cold water input to the hot water return with a T to send the water in the hot loop back through the boiler until an aquastat turns off the pump.
After I tested the pump and got water pumping through the system, the boiler would no longer sense water passing through to turn on the burner. The burner is operating fine, the boiler will stay warm if set for constant hot water, but the burner never goes to the code for heating flowing water, 04. Instead it cycles through the prep codes and then sits at 01. No call. This is while the water is running.
I replaced the flow sensor, thinking I had broken that, although the pump I used was the specified size in the installation manual for return loop piping.
I also replaced the fuse because I read that that Prestige will sometimes not cycle a call for heat when the fuse is blown--it is a ceramic fuse and hard to tell if it was or not. Neither helped.
Any other ideas?